1. Field of the Invention
This invention relates to a novel electrochromic and electrocatalytic material supported on a substrate, to a process for the preparation of the said material, to its use in electrochromic applications, and to its use as an electrocatalyst, more especially in alcohol fuel cells, in the electrochemical determination of alcohol contents and in the electrochemical oxidation of gaseous mixtures, more especially gaseous mixtures including carbon monoxide, for example, reformed gases containing hydrogen, carbon dioxide and carbon monoxide.
2. Brief Description of Related Art
Tungsten trioxide is an electrochromic material which colours by a reduction process. For applications to displays and windows, tungsten trioxide has been considered as a potential competitor to liquid crystals. However, there have been certain problems hindering its widespread acceptance, a crucial one being relatively slow response time arising from the fact that tungsten trioxide has relatively high resistivity and overpotential for hydrogen oxidation and evolution.
The reaction for the colouration of a tungsten trioxide film to hydrogen tungsten bronze in an acidic medium is as follows: EQU WO.sub.3 (colourless)+xH.sup.+ +xe.revreaction.H.sub.x WO.sub.3 (blue) where 0&lt;x.ltoreq.1.
The forward reaction is the colouring process and the backward reaction is the bleaching process. The colouring rate is limited by the diffusion rate of the hydrogen into the WO.sub.3 film to form the electronically conducting hydrogen tungsten bronze, as the initial reaction occurs at the interface of the conducting substrate and the film. If the highly resistive tungsten trioxide layer could be made more conductive, then this would have the effect of increasing the rate of colouration. One method of doing this is by the incorporation of catalysts into the film. It has previously been proposed to admix platinum black with tungsten trioxide powder, to bind the resulting mixture using a PTFE dispersion and to support the bonded mixture on a metallic screen. Use of such a Pt/WO.sub.3 mechanical admixture results in very active hydrogen evolution since the hydrogen evolution and oxidation overpotential on platinum is very low and the hydrogen formed readily diffuses into the tungsten trioxide lattice. The resulting hydrogen tungsten bronze will also participate in the electrochemical reaction, the so-called spill-over effect. Pt/WO.sub.3 mixtures, however, have the drawback that they rely on contacts between platinum particles and tungsten trioxide particles and not all of the platinum atoms can be utilised.